Chip-like electric components typically include a chip resistor, a chip inductor, a chip capacitor, and a chip-like composite electronic component formed by a combination of a plurality of types of electrical elements. Among the chip-like electric components are a chip-like electric component referred to as a multiple chip component of a multiple structure with a plurality of electrodes provided respectively on two opposed sides of an insulating substrate, in addition to a chip-like electric component having an electrode for soldering at each end of an insulating substrate.
Among terminal structures adopted in the conventional chip-like electric components, there is a terminal structure that uses a metal-glaze-based electrode containing silver. A configuration example of the terminal structure of this type will be described with reference to a terminal structure disclosed in Japanese Patent Application Publication No. 2002-237402 [Patent Document 1]. FIG. 3 is a vertical cross-sectional view showing a terminal structure of a publicly known chip-like resistor actually manufactured and marketed, based on Japanese Patent Application Publication No. 2002-237402. In the terminal structure of a chip-like resistor of this type, a metal-glaze-based front electrode 3 containing silver is provided on a front surface at an end portion of an insulating ceramic substrate 1. Further, a metal-glaze-based back electrode 5 containing silver is provided on a back surface at an end portion of the substrate. The front electrode 3 and the back electrode 5 of the paired structures are arranged to face each other with the insulating ceramic substrate 1 interposed therebetween. These metal-glaze-based front electrode 3 and back electrode 5 both containing silver are respectively formed by printing a metal glaze paste on the insulating ceramic substrate and then firing the printed metal glaze paste, for example. The metal glaze paste is formed by kneading Ag conductive powder or Ag—Pd conductive powder into a glass paste. A resistor layer 7 is formed on the front surface of the insulating ceramic substrate 1 as an electrical element forming layer electrically connected to the front electrode 3. Further, an electrically-insulating protective layer 9 made of an insulating material is formed to cover the overall resistor layer 7. The insulating protective layer 9 covers a portion of the front electrode 3, or partially covers the front electrode 3. The insulating protective layer 9 of this publicly known chip resistor is a two-layer structure formed of a glass layer 9a and an insulating resin layer 9b. In an actual product, the glass layer 9a is provided, covering a surface of the resistor layer 7 up to the top height or a peak of a raised portion 7a of the resistor layer 7 on an end portion of the front electrode 3, as illustrated. The insulating resin layer 9b is provided to cover a surface of the glass layer 9a as well as a surface of an end portion of the glass layer 9a, and also to partially cover the front electrode 3. At an end surface of the insulating ceramic substrate 1 where the front electrode 3 and the back electrode 5 are provided, a side electrode 11 is provided to electrically connect the front electrode 3 and the back electrode 5. The side electrode 11 is formed, using an Ag-resin-based conductive paint made by mixing silver into xylene-phenol resin or epoxy-phenol resin. Then, a conductive thin film layer 13 formed of two-plated layers is provided to cover an entire surface of the side electrode 11, also to cover an exposed portion of the front electrode 3, and to cover an entire back surface of the back electrode 5. The conductive thin film layer 13 is formed of a lower conductive thin film layer 13a and an external conductive thin film layer 13b. The lower conductive thin film layer 13a in this example is formed of a nickel-plated layer, while the external conductive thin film layer 13b is formed of a solder-plated layer.
No particular problem arises in the chip-like electric component having such a terminal structure as long as the chip-like electric component is used in an ordinary environment. However, it is known that when an electric apparatus including a circuit board with the chip-like electric component having this terminal structure mounted thereon is placed in an atmosphere rich in a sulfur component for a long time of period, a problem with electromigration arises.
More specifically, when the terminal structure of the chip-like electric component is exposed to the atmosphere containing sulfur (S) and moisture, the sulfur (S) enters into the component through an interface 15 where the insulating resin layer 9b and the conductive thin film layer 13 of the chip-like electric component meet each other with the moisture that has been condensed on the surface of the chip-like electric component working as a medium. In the conventional terminal structure of a chip-like electric component, the insulating resin layer 9b is overlapped with the conductive thin film layer 13 at the interface 15. However, the insulating resin layer 9b and the conductive thin film layer 13 are not physically or chemically combined. For this reason, it is considered difficult to completely block entry of electromigration-causing factors (e.g. moisture and sulfur). The electromigration-causing factors that have entered cause a sulfuration reaction with Ag in the front electrode 3, thereby producing silver sulfide (AG2S, or a tip-growth type whisker). More specifically, the sulfuration reaction proceeds as follows:Ag→Ag++e−S+2e−→S2−2Ag+S2−→AG2S
In order for this reaction to proceed, ionization of silver is necessary. Thus, the moisture becomes necessary. Once the sulfuration reaction has begun and the silver sulfide has been generated, the silver (Ag) contained in the front electrode 3 is then supplied to the tip of the whisker where Ag concentration is low. It means that silver (Ag) contained in the front electrode 3 comes out from the meeting surface between the insulating resin layer 9b and the conductive thin film layer 13. As a result, silver (Ag) contained in the front electrode 3 is reduced due to the sulfuration reaction. Accordingly, a resistance value of the front electrode 3 is increased, which finally causes a problem that the resistance value of the surface electrode 3 reaches to an open level at which disconnection occurs. Japanese Patent Application Publication No. 2002-237402 does not refer to countermeasures for preventing entry of the electromigration-causing factors into the front electrode 3 through the interface 15 between the insulating resin layer 9b and the conductive thin film layer 13.
Then, a technique is proposed for preventing entry of the electromigration-causing factors into the front electrode 3 through the interface 15 between the insulating resin layer 9b and the conductive thin film layer 13. The surface of the metal-glaze-based surface electrode 3 containing silver is partially covered with an end portion of the insulating resin layer 9b that covers the surface of the resistor layer 7. Then, other portions of the surface of the front electrode 3 that are not covered with the end portion are covered with the conductive thin film layer 13. In this condition, a resin-based conductive layer that does not contain silver (conductive layer formed of a paste in which conductive powder other than silver is mixed into a resin) is provided at a boundary between the surface of the front electrode 3 and the surface of the insulating resin layer 9b under the conductive thin film layer. The resin-based conductive layer that does not contain silver is intended to prevent entry of the electromigration-causing factors.
Japanese Patent Application Publication No. 2002-184602 (Patent Document 2), for example, discloses that a resin-based conductive layer that does not contain silver but contains nickel as conductive powder is used for this purpose. Further, Japanese Patent Application Publication No. 2004-259864 (Patent Document 3) discloses that a conductive resin paste that uses carbon as conductive powder is used to form a resin-based conductive layer that does not contain silver. Providing the resin-based conductive layer between the front electrode and the conductive thin film may suppress occurrence of electromigration and furthermore, may maintain electrical connection between the conductive thin film and the front electrode.
Each of Japanese Patent Application Publication No. 08-236302 (Patent Document 4) and Japanese Patent Application Publication No. 2002-25802 (Patent Document 5) shows that a resin-based conductive layer containing silver is provided on a front electrode. The Patent Document 4 shows that the resin-based conductive layer containing silver is formed on the front electrode in order to prevent a large level difference from being formed on the front electrode of a chip resistor (in order to planarize the surface of the chip resistor as much as possible). In a chip resistor disclosed in Japanese Patent Application Publication No. 2002-25802 (Patent Document 5), a resin-based conductive layer containing Ag which is highly heat-resisting is formed on the front electrode formed of an Au-based material, in order to protect the front electrode from heat of soldering. Either of these patent documents does not refer to the anti-electromigration performance of the resin-based conductive layers containing Ag. However, WIPO International Publication No. WO2003/046934 (Patent Document 6) cites Japanese Patent Application Publication No. 08-236302 (Patent Document 4) as a conventional art and describes that corrosion is caused due to migration (electromigration) even when the resin-based conductive layer containing silver is provided on the front electrode as shown in Patent Document 4. For this reason, Patent Document 6 teaches that a glass overcoat is formed over a glass cover coat so that the glass overcoat covers a boundary portion between the resin-based conductive layer containing silver and the glass cover coat formed over a resistor body. Covering the boundary portion with the overcoat is intended to prevent occurrence of the electromigration.
Japanese Patent Application Publication No. 2002-64003 (Patent Document 7) shows that a silver-based thick film (a conductive layer containing silver) is provided between a front electrode and a protective layer that covers a resistor body. The silver-based thick film contains 5% or more of palladium, and a rest of the film is formed of silver and a resin. Patent Document 7 shows that the silver-based thick film containing 5% or more of palladium has an excellent anti-electromigration property. In the structure disclosed in Patent Document 7, however, an interface between the protective layer that covers the resistor body and a plated layer, and a short interface between the protective layer and the silver-based thick film, formed continuously with the interface between the protective layer and the plated layer, extend to the front electrode which is not covered with the silver-based thick film. Patent Document 7, in particular, describes that when the silver-based thick film containing 5% or more of palladium is formed on a part of the front electrode (an upper surface electrode), cost may be reduced more, compared with when the front electrode is entirely formed of a silver-based thick film containing palladium and having an excellent anti-electromigration property. Judging from this description, it is presumed that by reducing an amount of the silver-based thick film containing 5% or more of palladium as much as possible, the interface between the protective layer and the silver-based thick film described above will be considerably short in length.
Japanese Patent Application Publication No. 07-169601 (Patent Document 8) shows that a second upper surface electrode layer is provided to extend over an overcoat glass layer on a resistor layer. Since the second upper electrode layer is fired at 600° C., the second upper electrode layer is a metal glaze paste containing silver rather than a resin paste containing silver.
Japanese Patent Application Publication No. 07-302510 (Patent Document 9) discloses a glass-based conductive paste material used for forming a metal-glaze-based electrode rather than a resin electrode. As a conductive constituent of a conductive paste composition, nickel is contained in addition to silver. The conductive paste material contains three types of conductive powder including fine spherical silver powder, coarse spherical silver powder or coarse spherical silver-coated nickel powder, and flake-like silver powder.
Japanese Patent Application Publication No. 2001-126901 (Patent Document 10) shows a configuration of a chip resistor: wherein an upper surface electrode layer is provided on an end portion of an insulating substrate; a resistor layer is provided on the insulating substrate, overlaying the end portion of the upper surface electrode layer; and a protective layer formed of a glass layer alone is provided, covering an entire surface of the resistor layer and partially covering the upper surface electrode layer; a side electrode layer formed of a silver-based thick film or a resin-silver-based thick film is provided, covering the end portion of the protective layer formed of the glass layer alone and an exposed portion of the upper surface electrode layer; and a plated layer is provided, covering a surface of the side electrode layer and the protective layer formed of the glass layer alone. In this chip resistor, the overlap length between the insulating layer and the conductive layer that blocks entry of the electromigration-causing factors is specifically defined.
[Patent Document 1] Japanese Patent Application Publication No. 2002-237402, FIG. 2
[Patent Document 2] Japanese Patent Application Publication No. 2002-184602, FIG. 1
[Patent Document 3] Japanese Patent Application Publication No. 2004-259864, FIG. 1
[Patent Document 4] Japanese Patent Application Publication No. 08-236302, FIG. 1
[Patent Document 5] Japanese Patent Application Publication No. 2002-25802, FIG. 1
[Patent Document 6] WIPO International Publication No. WO2003/046934, FIG. 2
[Patent Document 7] Japanese Patent Application Publication No. 2002-64003, FIG. 1
[Patent Document 8] Japanese Patent Application Publication No. 07-169601, FIG. 1
[Patent Document 9] Japanese Patent Application Publication No. 07-302510
[Patent Document 10] Japanese Patent Application Publication No. 2001-126901, FIG. 1